Does federal law prohibit ex parte communications from private parties to administrative agency employees who are deciding whether to adopt a rule during notice-and-comment rulemaking proceedings?

Yes, because a reviewing court must have an adequate record before deciding whether an agency had adequate support for its rule.

No, because section 53 is silent on the question of ex parte contracts.

Yes, but only when the rulemaking resolves competing claims among a small group of claimants to a valuable governmental benefit.

No, so long as the agency can demonstrate that the comment dealt only with policy questions and did not introduce new data.